The present invention relates to a process and an apparatus for the production of an extruded, stretched, seamless tube from partially crystalline polymers, which is continuously calibrated on the outside in one operation.
A process for the production of a parison, in which an outer calibration of the parison takes place in a vacuum, is known from U.S. Patent Specification No. 3,804,574. Outer calibration displays advantages over inner calibration, in particular for tube diameters of less than 70 mm. Due to a relatively small transverse stretching ratio of .lambda..sub.Q =3.4, the linkages belonging to a mandrel of less than 24 mm diameter can no longer be accommodated (in the case of inner calibration) in a torpedo bore of a die of less than 24 mm diameter. Since the linkage must include a stretching-air supply line, a water feed, a water return and a purging-air supply line, it is obvious that the linkage diameter cannot be reduced to a diameter of less than 24 mm.
An apparatus for the production of an extruded tube from isotactic polypropylene with a calibrating device which is arranged on the extrusion head of an extruder inside the tube and consists of plates arranged perpendicular to the tube axis, is known from German Patent Specification No. 1,242,851. A tubular casing encloses the tube and is arranged in a cooling bath underneath the surface of the cooling liquid. Cooling liquid flows over the upper rim of the casing onto the outer surface of the tube. In the lower region, the casing is sealed off against the admission of cooling liquid and has a connection which is connected to a suction pump delivering the cooling liquid into the storage tank. Through the calibrating die there leads a compressed-air supply line for regulating the volume of the tube in the piece between the calibrating die and rolls which fold up the tube inside the collecting tank and deflect it upward for exiting the collecting tank. With this apparatus, an inner calibration of the tube takes place.
The apparatus described in German Auslegungsschrift 1,504,050 for the production of biaxially-oriented thermoplastic films in the blow-molding process likewise has an inner calibrating device of one or more plates, which bound a cooling zone containing a cooling medium, which is closed off at the opposite end by a seal. The mandrel protruding from the extrusion die into the tube contains three mutually coaxial pipes for the supply and discharge of cooling liquid into the cooling zone and the air supply into the tube underneath the seal.
In German Offenlegungsschrift No. 2,136,168, a process and an apparatus for the production of a continuously extruded tube are described, the tube being led from an extrusion die of an extruder vertically downward into a quenching bath, while the inside of the tube is cooled by a cooling liquid which is circulated in the tube. The inner calibration takes place by means of a plurality of calibrating plates, which are arranged coaxially to one another and perpendicular to the axis of the tube. The pressure of a gas, which is introduced via a line into the tube above the calibrating plates, is regulated with the aid of valves. The tube runs vertically downward via the calibrating plates to a pair of pinch rolls, which smooths the tube over part of its width. Then the tube is deflected upward out of the quenching bath and subsequently wound up or subjected to further processing.